A known differential gear mechanism disclosed in JP8-268099A (corresponding to U.S. Pat. No. 5,735,766), for example, includes a housing rotationally driven about a rotational axis thereof, a pair of sun gears coaxial with the rotational axis and including external gear portions at outer peripheral surfaces, respectively, and a pair of planetary gears rotatably received in the housing and engaging with the pair of sun gears, respectively. The pair of planetary gears also engages with each other. The sun gears are in connection with output shafts, respectively, in a non-rotatable manner. Accordingly, when the housing is driven to rotate by an engine, the rotational driving force (i.e., torque) supplied by the engine to the housing is transmitted to the sun gears and further to left and right wheels via the respective output shafts.
The external gear portions of the sun gears and the planetary gears include helical teeth, respectively. Thus, a thrust force is generated at an engagement portion between each of the external gear portions of the sun gears and each of the planetary gears. Because of the thrust force, the sun gears are subjected to force in the direction of the rotational axis of the sun gears. An axial end surface of each of the sun gears is pressed against each contact surface provided inside of the housing. Accordingly, at the time of differential rotation of the sun gears, a frictional resistance is generated between the end surface of each of the sun gears and the contact surface of the housing for restricting the rotation of the sun gears. The differential rotation of the sun gears is limited because of the frictional resistance.
These days, in order to improve traveling ability of a vehicle, an increase of differential limiting force is desired. In responding to such desire, it is considered to increase helical angles of the external gear portion and the planetary gear for the purpose of increasing the thrust force generated at the engagement portions therebetween. The larger the thrust force is, the greater the friction resistance is generated between the contact surface of the housing and the end surface of each of the sun gears, thereby increasing the differential limiting force. However, increasing the helical angle of each of the gears more than a certain level is difficult in view of strength, and the like. Therefore, the increase of the thrust force is limited to a certain extent, which results in difficulty in increasing the differential limiting force.